Selective non-catalytic reduction of NOx

ABSTRACT

A non-catalytic process for NO x  concentrations in process streams wherein a reducing agent and a readily-oxidizable gas are injected into a NO x -containing process stream to reduce the concentration of the NO x -containing process stream while minimizing the carryover of the unreacted reducing agent.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/427,265 filed May 1, 2003, which claims benefit of thefollowing U.S. Provisional Patent Applications: Ser. No. 60/386,560filed Jun. 5, 2002; Ser. No. 60/386,492 filed Jun. 5, 2002; and Ser. No.60/442,268 filed Jan. 24, 2003.

FIELD OF THE INVENTION

The present invention relates to a non-catalytic process for reducingNO_(x) concentrations in process streams. More particularly, the presentinvention relates to the injection of a reducing agent in combinationwith a readily-oxidizable gas to reduce NO_(x) emissions in processstream effluents.

BACKGROUND OF THE INVENTION

Increasingly stringent government regulatory emission standards haveforced refiners to explore, and in some cases to implement, improvedtechnologies for reducing the concentration of nitrogen oxides (NO_(x))in emissions from combustion and production effluent streams. Forexample, it is known in the art to reduce NO_(x) concentrations incombustion effluent streams by the injection of ammonia, and one suchpatent covering this technology is U.S. Pat. No. 3,900,554 to Lyon,which is incorporated herein by reference. After this Lyon patent, therewas a proliferation of patents and publications relating to theinjection of ammonia into combustion effluent streams in order to reducethe concentration of NO_(x). Such patents include U.S. Pat. Nos.4,507,269, Dean et al., and 4,115,515, Tenner et al., both of which arealso incorporated herein by reference. Other patents disclose the use ofammonia injection based on the use of kinetic modeling to determine theamount of ammonia to be injected. Such patents include U.S. Pat. Nos.4,636,370, 4,624,840, and 4,682,468, all to Dean et al., and all ofwhich are also incorporated herein by reference. There have also been anumber of patents and publications relating to the injection of ureainto combustion effluent streams in order to reduce the concentration ofNO_(x). One such patent covering this technology is U.S. Pat. No.4,208,386 to Arand et al., which is incorporated herein by reference. Astudy by Kim and Lee (1996), incorporated herein by reference, publishedin the Journal of Chemical Engineering of Japan shows that ureadissociates to ammonia and cyanuric acid (HNCO) and that both of theseact as reducing agents for NO in two interrelated chains of free radicalreactions.

However, effluents released from process streams remain a source ofNO_(x). Particularly troublesome NO_(x) pollutants found in many processeffluent streams are NO and NO₂. The NO_(x) emissions rates forindustrial processes are regulated in the United States by theEnvironmental Protection Agency (“EPA”) and other local environmentalagencies. NO₂ is a major component of smog and NO is readily convertedto NO₂ when exposed to sunlight in the presence of O₂. Some of the majorindustrial processes contain large amounts of NO in the process streams.

Examples of such process streams that are a source of NO_(x) include theoff-gas stream from the regenerator of a fluidized catalytic crackingunit, (“FCCU”) and a carbon monoxide combustion/heat recovery unit(COHRU) used in conjunction with a FCCU. In particular, the FCCU processtypically generates significant amounts of NO_(x) wherein most of theNO_(x), often in the range of 90% of the total NO_(x), is in the form ofNO. This NO in the process stream is particularly difficult to removewithout first converting the NO to elemental nitrogen or higher nitrogenoxide species. NO_(x) in an FCC off-gas effluent results from theburning of carbon deposits from the spent catalyst. However, it isdifficult to burn the carbon deposits from a spent catalyst withoutgenerating NO_(x) in the off-gas. NO_(x) produced in the regenerator andpresent in the off-gas is typically passed to the COHRU, which convertsCO in the FCCU regenerator off-gas to CO₂ and other products such aswater and/or steam. As the COHRU converts CO to CO₂ and other products,the effluent emitted into the atmosphere also contains NO_(x). It isdifficult to reduce the NO_(x) concentrations in these streams bythermal means, partially because of the low temperatures of theseprocess streams. Some catalyst fines may also be present in theregenerator off-gas. The effect of catalyst fines on NO_(x) reductionwas demonstrated at temperatures below 850° F. in U.S. Pat. No.4,434,147, Dimpfl et al., incorporated herein by reference. The '147patent describes a process in which ammonia and FCCU regenerator off-gasare cooled, then passed through a bed of FCCU catalyst fines created bycollecting the fines on specially adapted electrostatic precipitatorplates.

Hydrogen (H₂) injection has been utilized in the past to enable thenon-catalytic, ammonia-based, NO_(x) reduction process to be moreeffective with lower temperature combustion effluent streams. Whilehydrogen injection has been used before with ammonia to reduce NOx inlow temperature combustion streams, the amount of NO_(x) released to theatmosphere is still too high for more stringent environmentalregulations. Therefore, there exists a need in the art for improvedmethods of reducing the emission of NO_(x) in refinery process streamsby non-catalytic means. Current NO_(x) emission limits depend on siteregulations, but are commonly in the range of 50 ppm for an average FCCunit. Due to the relatively large volume of NO_(x) emitted per year bysuch NOx producers as an FCC unit, even small decreases in the parts permillion levels of NO_(x) emitted from these units amounts to tons ofpollutant emissions that can be eliminated in a single year from such aunit. Many of these units are operating at the limits of current costeffective technology for these high-volume NO_(x) generating units.Therefore, improvements even as small as a 5-10 vppm reduction in NO_(x)emissions on a commercial FCC unit are significant in light of thecurrent art.

Another problem that exists in the art is that non-catalytic, ammonia(NH₃) based NOx reduction processes, such as disclosed U.S. Pat. No.3,900,554 to Lyon, can result in significant amounts of ammonia in thetreated gas stream. This can result in excessive ammonia emissions andin certain applications, as in an FCCU, a significant portion of theunreacted ammonia converts into ammonium salts which can be corrosiveand which tend to deposit on related downstream equipment, such as heatexchange equipment, turbines, scrubber slurry lines, environmentalanalyzer sample systems, etc. These corrosive and restrictive saltdeposits can cause significant equipment deterioration and systemfailures. Additionally, in an FCCU, much of the ammonia left in thetreated streams is removed via FCCU waste or recycle streams to therefinery's waste water system for treatment. These sophisticated modernwater treatment systems normally include treatment plant bioreactorswhich are an essential component of the waste water treatment processesrequired to meet strict EPA water quality guidelines. Here, in thesebioreactors, the ammonia can kill the biological matter upon which themodern waste water treatment plant relies on to break down the organicpollutants to meet these mandated clean water discharge permitlimitations.

In addition, if excess NH₃ is utilized in these processes (resulting in“ammonia slip”), the NH₃ reaching downstream combustion equipment willoxidize to NO_(x), decreasing the net NO_(x) reduction achievable viathese processes. Therefore, in processes where there is combustionequipment downstream of an ammonia (NH₃) based NO_(x) reduction process,it is particularly important that the ammonia slip be controlled to lowlevels. A problem with the processes existing in the art is being ableto simultaneously achieve both high levels of NO reduction and lowlevels of downstream unreacted ammonia concentrations.

Therefore, there exists in the art the need for an economic,non-catalytic process with improved NO_(x) emissions reductioncapabilities. In addition, there exists a need in the art for aneffective NO_(x) removal process with low levels of unreacted ammoniawhich is detrimental to air quality emissions, water quality emissions,and associated equipment reliability, longevity, and functionality.

SUMMARY OF THE INVENTION

In one embodiment, the present invention provides a non-catalyticprocess for reducing the NO_(x) concentration in the regenerator off-gasof a fluid catalytic cracking unit, comprising:

-   -   a) forming a mixture of a reducing agent selected from ammonia,        urea and mixtures thereof, and a first readily-oxidizable gas in        effective amounts that will result in the reduction of the        NO_(x) concentration of the regenerator off-gas by a        predetermined amount;    -   b) injecting said mixture into said regenerator off-gas at a        first injection point wherein the regenerator off-gas is at a        temperature between about 1200° F. and 1600° F.; and    -   c) injecting a second readily-oxidizable gas at a second        injection point downstream of the first injection point in an        amount effective to further reduce the amount of NO_(x)        concentration of the regenerator off-gas and to reduce the        concentration of the reducing agent in the regenerator off-gas.

In another embodiment, the reducing agent is injected in a molar ratioof about 1 to about 10 moles per mole of NO.

In still another embodiment, the mixture injected into the firstinjection point is comprised of the first readily-oxidizable gas and thereducing agent in a molar ratio of about 1 to about 20 moles ofreadily-oxidizable gas per mole of reducing agent.

In still another embodiment, the second readily-oxidizable gas isinjected in a molar ratio of about 1 to about 40 moles of secondreadily-oxidizable gas per mole of unreacted reducing agent from saidfirst injection point.

In a preferred embodiment, the final NO_(x) concentration of saidregenerator off-gas at a point downstream of said second injection pointis less than 50 vppm.

In still another preferred embodiment, the reduction of the NO_(x)concentration of said regenerator off-gas is greater than 50 vol %.

In a more preferred embodiment, the reducing agent is ammonia and theconcentration of the ammonia by vol % of the regenerator off-gas afterthe second injection point is at least 60% lower than the concentrationof the ammonia by vol % of the regenerator off-gas between the firstinjection point and the second injection point.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a highly simplified schematic of a commercial FCCUregenerator off-gas circuit similar to the circuit from which the testdata included herein was obtained.

FIG. 2 shows a plot of the NO_(x) concentration in the FCCU regeneratoroff-gas stream as a function of the NH₃/NO ratio of the first injectionpoint at varying H₂/NH₃ ratios. The NO_(x) baseline readings for theregenerator off-gas stream without H₂/NH₃ injections are also shown onthe plot for similar timeframes. This data was obtained from injectionof hydrogen and ammonia at a single injection point into the regeneratoroff-gas stream of a commercial fluidized catalytic cracking unit.

FIG. 3 shows a plot of the NO_(x) concentration in the FCCU regeneratoroff-gas stream as a function of the NH₃/NO ratio of the first and secondinjection points at varying H₂/NH₃ ratios. The NO_(x) baseline readingsfor the regenerator off-gas stream without H₂/NH₃ chemical injectionsare also shown. This data was obtained from the injection of hydrogenand ammonia at a first injection point into the regenerator off-gasstream of a commercial fluidized catalytic cracking unit followed by theinjection of hydrogen at a second injection point into the regeneratoroff-gas stream.

FIG. 4 shows a plot of the same data as shown in FIG. 3, wherein thedata was averaged and reformatted to more clearly illustrate the effectson NO_(x) as a function of NH₃/NO ratio for varying first and secondinjection point H₂/NH₃ ratios.

FIG. 5 shows a plot of the NH₃ concentration in the FCCU regeneratoroff-gas stream as a function of the NH₃/NO ratio for varying first andsecond injection points at varying H₂/NH₃ ratios. The NH₃ baselinereadings for the regenerator off-gas stream without H₂/NH₃ chemicalinjections are also shown. This data was obtained from the injection ofhydrogen and ammonia at a first injection point into the regeneratoroff-gas stream of a commercial fluidized catalytic cracking unitfollowed by the injection of hydrogen at a second injection point intothe regenerator off-gas stream.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

As used herein, the reference to NO_(x), or nitrogen oxide(s) refers tothe various oxides of nitrogen that may be present in process streamssuch as, for example, the off-gas of the regenerator of a fluidizedcatalytic cracking unit. Thus, the terms refer to all of the variousoxides of nitrogen including nitric oxide (NO), nitrogen dioxide (NO₂),nitrous oxide (N₂O), etc. and mixtures thereof.

Mixing, as used herein when describing the mixing of the reducing agentand readily-oxidizable gas, is meant to refer to the broadest meaninggiven the term. Thus, mixing refers to the objective of maximizing thelocal contact of the reducing agent and readily-oxidizable gas with theNO_(x) in the process stream at the desired molar ratios. Any suitablemixing techniques can be employed to achieve this end. These techniquesinclude, but are not limited to, using a carrier gas with the reducingagent and/or readily-oxidizable gas to encourage more homogenous mixing;injecting a premixed stream of a reducing agent, readily-oxidizable gasand carrier gas into the process stream; or, injecting a stream ofreducing agent and carrier gas and a stream of readily-oxidizable gasand carrier gas into the process stream separately.

Non-limiting examples of suitable pre-injection mixing techniques,processes or means include piping the reducing agent, readily-oxidizablegas and carrier gas through separate lines into one common vessel orinto the injection line to the process stream to be treated, allowingthe two reagents and the carrier to mix as they flow towards theinjection point.

An embodiment is a non-catalytic process that uses an effective amountof a reducing agent injected with an effective amount of areadily-oxidizable gas to reduce the NO_(x) concentration of a processstream preferably by a predetermined amount or to below a specifiedconcentration limitation. By a predetermined amount it is meant areduction of NO_(x) by more than about 30% by volume, and preferablymore than about 50% by volume, based on the total volume of NO_(x)present in the process stream. By the term a specified concentrationlimitation, the final NOx concentration limitation or target should beexpressed in vppm (parts per million by volume) of NO_(x). For an FCCregenerator off-gas stream, this limitation is preferably 50 vppm, morepreferably 40 vppm, and even more preferably 30 vppm. In a mostpreferred embodiment, the predetermined reduction of NO_(x) or NO_(x)limitation is at least that amount sufficient to meet governmentalregulatory emission standards.

The present process is suitable for treating any process streamcontaining NO_(x) and greater than about 0.1 vol. % oxygen, based on thevolume of the stream. Preferably the stream will contain about 0.4 toabout 1.5 vol. % oxygen, although embodiments including about 0.1 toabout 3.0 vol. % oxygen are contemplated to be within the scope of theprocess described herein. The present process is especially well-suitedfor treating the regenerator off-gas of a fluidized catalytic crackingunit.

Fluidized catalytic cracking is an important and widely used refineryprocess. The catalytic cracking process typically converts heavy oilsinto lighter products such as gasoline. In the fluidized catalyticcracking (FCC) process, an inventory of particulate catalyst iscontinuously cycled between a cracking reactor and a catalystregenerator. Average reactor temperatures are in the range of about900-1000° F., with average feed temperatures from about 500-800° F. Thereactor and the regenerator together provide the primary components ofthe catalytic cracking unit. FCC process units are well known in the artand U.S. Pat. No. 5,846,403, Swan, et al., incorporated herein byreference, provides a more detailed discussion of such a unit.

The regenerator is especially important to catalyst life andeffectiveness because during the fluidized catalytic cracking process,carbonaceous deposits (coke) are formed on the catalyst, whichsubstantially decrease its activity. The catalyst is then typicallyregenerated to regain its effectiveness by burning off at least aportion of the coke in the regenerator. This is typically done byinjecting air, or another gas having a combustible amount of oxygen,into the regenerator at a rate sufficient to fluidize the spent catalystparticles. A portion of the coke contained on the catalyst particles iscombusted in the regenerator, resulting in regenerated catalystparticles. Typical regenerator temperatures range from about 1200° F. toabout 1600° F.

After regeneration, the catalyst particles are cycled back to thereactor. The regenerator off-gas is usually passed to further processessuch as heat recovery devices, particulate removal devices, and carbonmonoxide combustion/heat recovery units (COHRU), which, as previouslymentioned, are designed to convert CO to CO₂ and recover available fuelenergy. However, the specific equipment and/or equipment configurationassociated with an FCCU regenerator off-gas system may vary in designfrom installation to installation.

However, regardless of the equipment configuration, it is difficult toburn a substantial amount of coke from the catalyst in the regeneratorwithout increasing the NO_(x) content of the resulting off-gas.Therefore, the regenerator off-gas will typically contain nitrogenoxides (NO_(x)), catalyst fines, sulfur oxides (SO_(x)), carbon dioxide,carbon monoxide, and other compounds formed during the combustion of atleast a portion of the coke from the catalyst particles. Of the nitrogenoxides present in the regenerator off-gas, nitric oxide (NO) typicallymakes up the majority of all NO_(x) present. NO will usually representabout 90% of the total NO_(x) in the regenerator off-gas. Therefore, thepresently claimed process is especially concerned with the reduction andcontrol of NO.

It may be preferred to operate the regenerator in full burn mode to burncoke from the catalyst. During full-burn mode, the regenerator off-gascomposition is generally about 0.6-1.5 vol. % oxygen, about 15-20 vol. %water, about 50 to about 200 parts per million by volume (vppm) NO,about 20-50 vppm CO, about 500-1000 vppm SO₂ with the balance being N₂and CO₂.

Concentrations of NO_(x) in process streams can be reduced by 50 vol. %or more through the use of the present non-catalytic NO_(x) reductionprocess. This is well within the desired reduction range describedabove. The only commercially available technology to achieve such alevel of reduction is technology based on the use of a catalyticprocess, which is significantly more expensive when compared to thepresent non-catalytic process.

The present invention, however, achieves NO_(x) reductions in lowtemperature process streams by the injection of a reducing agent. Theprocess streams treated with the presently claimed process alsotypically have low concentrations of oxygen, necessitating the use of areadily-oxidizable gas being injected with the reducing agent.

Reducing agents suitable for use in the presently claimed inventioninclude urea, ammonia, and mixtures thereof. The preferred reducingagent is ammonia. Readily-oxidizable gases suited for use in the presentprocess include paraffinic, olefinic and aromatic hydrocarbons andmixtures thereof such as, for example, gasoline and fuel oil, oxygenatedhydrocarbons including formic and oxalic acids, nitrogenatedhydrocarbons, sulfonated hydrocarbons, carbon monoxide, and hydrogen.Hydrogen is the preferred readily-oxidizable gas since it is not itselfan air pollutant and cannot yield an air pollutant by incompleteoxidation.

By injection, it is meant that the readily-oxidizable gas, the reducingagent, or combinations thereof are conducted or introduced into theNO_(x) containing process stream to be treated. This injection may beperformed by any suitable means known in the art. The injection meanschosen is not critical to the present invention as long as it is onethat effectively introduces the reducing agent and/or readily-oxidizablegas into the process stream for adequate contact and mixing.

An effective amount of reducing agent used herein is based on the amountof NO_(x) that is to be reduced. The amount of reducing agent used willtypically range from about 1 to 10 moles of reducing agent per mole ofNO_(x), preferably about 3 to 8 moles of reducing agent per mole ofNO_(x). The measurement of the concentration of NO_(x) in theregenerator off-gases may be achieved by any suitable method known inthe art, and the method chosen is not critical to the process presentlyclaimed.

It is believed that a complex chain of free radical reactions achievesthe non-catalytic reduction of NO_(x) with the present reducing agentand readily-oxidizable gas. Not wanting to be limited by theory, theinventors herein believe the overall effect can be illustrated by thefollowing two competing reactions:NO+NH₃+O₂→N₂+H₂O (reduction)  Equation 1NH₃+O₂→NO+H₂) (oxidation)  Equation 2

The use of urea as the reducing agent introduces cyanuric acid (HNCO) aswell as ammonia to the process. As demonstrated in the work of Lee andKim (1996), cyanuric acid acts as a reducing agent for NO and alsointeracts with the NO—NH₃—O₂ chemistry summarized in Equations 1 and 2.Although the cyanuric acid reduction process is not thoroughlyunderstood, and not wishing to be limited by theory, the inventorshereof believe that the dissociation of one mole of urea liberates onemole of ammonia and one mole of cyanuric acid. Experimental data fromthe Kim and Lee study (1996) suggests that cyanuric acidstoichiometrically reduces NO to elemental nitrogen and water at a molarratio with NO of 1:1. Thus, urea should generally be used at a molarratio to NO that is roughly one half the effective molar ratio forammonia.

The reduction reaction of Equation 1 dominates in the 1600° F.-2000° F.temperature range. Above 2000° F., the reaction of Equation 2 becomesmore prevalent. Thus, in the practice of the present invention, it isdesirable to operate at temperatures below about 2000° F. However,operating temperatures lower than about 1600° F. are achievable with thereduction reaction still being dominated by Equation 1 through the useof the present invention. It has been found herein that, at temperaturesbelow about 1600° F., the reduction reaction of Equation 1 will noteffectively reduce NO_(x) without the injection of a readily-oxidizablegas, such as hydrogen. However, it has also been found that a singleinjection point containing the reducing agent and the readily-oxidizablegas cannot achieve the significantly improved NO_(x) reduction that maybe achieved by utilizing a second injection point. In order to achievesignificant NO_(x) reduction improvements over a single injection point,it has been discovered that a readily-oxidizable gas must be injected ata second injection point downstream of the first injection point in veryspecific ratios.

Additionally, it has been found that in order to maximize the reductionof NO_(x) that the NH₃/NO_(x) ratio needs to be above about 3. However,it has been found that these limitations are very narrow and that thereare no improvements of NO_(x) reduction above NH₃/NO_(x) ratios of about8. However, applicants have found that at NH₃/NO_(x) ratios above about2, significant amounts of NH₃ remain in the process stream. As statedbefore, this can cause significant environmental and equipment problems.It has been unexpectedly discovered that by injecting a high volume ofreadily-oxidizable gas at a second injection point downstream of thefirst injection point, that the ammonia content in the final processstream can be significantly reduced. As are shown in Examples 1-2 andFIGS. 2-5, by introducing a second stream of a readily-oxidizable gas inspecific ratios, significant reductions in NO_(x) and NH₃ emissions canbe achieved over the prior art.

It should be noted that as the temperature of the process streamdecreases, the amount of readily-oxidizable gas needed to drive thereduction reaction increases. However, it has been determined hereinthat by injecting specific molar ratios of a reducing agent and areadily-oxidizable gas at a first injection point coupled with asecondary injection of a readily-oxidizable gas further downstream asdisclosed herein can be used at an effective operating temperature rangebelow about 1600° F. This makes the present embodiment especially suitedfor reducing NO_(x) concentrations in the off-gas of an FCCU regeneratorbecause the temperature of the regenerator off-gas stream is typicallylow, below about 1600° F. It should be noted, however, that the presentembodiments can also effectively operate over any temperature rangebetween about 1200° F. to about 1600° F.

FIG. 1 shows a highly simplified schematic of an FCC unit regeneratoroff-gas configuration. This sketch eliminates many of the extraneouscircuits and equipment associated with the operation of such a complexunit. This figure shows a carbon monoxide combustion/heat recovery unit(COHRU) downstream of the regenerator followed by a wet gas scrubbersystem. Most FCC units also have additional catalyst particulate removalunits in this circuit. Other FCC units may not have a COHRU, but insteadhave a waste heat recovery boiler and may also utilize a gas expanderfor energy recovery. However, the present embodiments apply to all suchconfigurations. But, as this illustrates, most FCC units containsignificant expensive machinery, including catalyst disengagingequipment, rotating equipment, heat exchangers, emissions treatingequipment, and analyzer and controls equipment that can be damaged bythe corrosive agents (such as NO_(x) and ammonia salts) that the FCCUprocess and the NO_(x) reduction processes of the prior art cangenerate.

Returning to FIG. 1, the FCC unit regenerator (1), produces aregenerator off-gas (2) in the range of about 1200 to about 1600° F.This gas contains a significant amount of steam and carbon dioxide,oxygen, and unwanted contaminants such as carbon monoxide, nitrousoxides, sulfur oxides, and ammonia. In the present invention, a reducingagent (3) and a readily-oxidizable gas (4) are injected at a firstinjection point (6) into the regenerator off-gas. Optionally, a carriergas (5), such as steam or air, may be utilized to help increase thevelocity of the injection stream and improved mixing and dispersion ofthe stream within the regenerator off-gas stream. The readily-oxidizablegas (4) is also injected at a second injection point (7) downstream ofthe first injection point. While it is not known exactly how fardownstream the second injection point must be located with respect tothe first injection point, the second injection point should be farenough downstream to prior allow the reaction of reducing agent and thereadily-oxidizable gas from the first injection point with theregenerator off-gas to reach near equilibrium.

Continuing with FIG. 1, the treated regenerator off-gas (8) proceedsthrough a carbon monoxide combustion/heat recovery unit (COHRU) (9)wherein the carbon monoxide (CO) is thermally reacted to convert themajority of the CO to carbon dioxide (CO₂). The off-gas stream is thenrouted to a was gas scrubber (WGS) (10) where the regenerator off-gasundergoes an additional liquid scrubbing treatment, normally for theremoval of particulates and SO_(x), where after the treated off-gas isdischarged to the atmosphere (11). On-line analyzers may be mayoptionally be placed at following locations to measure certain streamcomposition characteristics for analysis: at the regenerator outlet(12), between the first injection point and the second injection point(13), and downstream of the second injection point (14). These on-lineanalyzers can be for data collection only or can be used to providefeedback to varying controls systems to either automatically control theinjection rates and/or compositions of the injection streams of thepresent invention, or to control other aspects of the FCCU process.These analyzers may detect concentrations such as, but not limited to,NO_(x), SO_(x), ammonia, hydrogen, carbon monoxide, and carbon dioxide.As part of this invention, it should be noted that it is not necessaryto have any or all of the on-line analyzers so listed, althoughinformation obtained from such analyzers can be beneficial in optimizingthe control of the injection systems of the present process. It shouldalso be noted that additional analyzers may be located at other pointsin the FCC unit regenerator off-gas circuit depending upon whichmolecular compounds are to be measured or which aspects of the processare being controlled as a result of the stream analysis.

The data presented in FIGS. 2 through 5 were obtained through tests on acommercial FCCU regenerator off-gas stream similar to as shown in thesimplified depiction of FIG. 1. The data included in these figures andthe associated analyses are further detailed in Examples 1 and 2.

A readily-oxidizable gas is used to induce the NO_(x) reductionreaction. An effective amount of readily-oxidizable gas is that amountthat enables the reducing agents of the present invention to effectivelyreduce the NO_(x) concentration by a determined amount. A molar ratio ofabout 1 to about 20 moles of a first readily-oxidizable gas per mole ofreducing agent is considered an effective amount of firstreadily-oxidizable gas. Preferably this molar ratio is 1 to about 8,more preferably about 1 to about 3. The actual molar ratio employed willbe dependent on such things as the temperature of the process stream;the composition of the process stream; the effectiveness of theinjection means used for mixing the readily-oxidizable gas with thecarrier gas, the reducing agent and the NO_(x)-carrying stream; and thereducing agent utilized. Thus, for a given process stream, the mosteffective readily-oxidizable gas to reducing agent molar ratio may lieanywhere within a range of about 1 to about 20 moles of a firstreadily-oxidizable gas per mole of reducing agent range.

A readily-oxidizable gas is required for the reduction reaction of NOand the reducing agent at lower temperatures (typically below about1600° F.) and at process low oxygen levels (typically below 3.0 vol. %oxygen). Most typical refinery process streams, including the FCC unitregenerator off-gas, have low oxygen levels, thus necessitating the needfor the use of readily-oxidizable gas in combination with the reducingagent and the process stream in order to achieve a considerable NO_(x)reduction. In addition, low temperatures in the process stream (<1600°F.) also contribute to the need for a readily-oxidizable gas. However,it has been discovered that there are limitations that may be achievableby a single injection of a readily-oxidizable gas and a reducing agentin achieving adequate NO_(x) reduction in these process streams.

It has also been discovered that there is a limitation to the amount ofreducing agent that can properly react with the NO in the process gasregardless of the amount necessary to reduce the NO to elementalnitrogen. This can be seen in FIG. 2, wherein overall NO_(x) reductionwith a single injection point does not improve with NH₃/NO ratiosgreater than about 8. Also referring to FIG. 2, final average NO_(x)concentrations below 40 to 60 vppm could not be achieved regardless ofthe H₂/NH₃ ratio employed.

It has been unexpectedly discovered that by additionally introducing areadily-oxidizable gas at a second point at an effective distancedownstream of the first injection point that NO_(x) levels can bereduced to levels on the order of 25% to 50% lower than achievable bythe use of a readily-oxidizable gas and a reducing agent in a singleinjection point. These results can be seen in FIG. 3, where finalaverage NO_(x) levels of 20 to 40 vppm were achieved by the presentinvention. It should be noted here that these levels of NO_(x) reductionare extremely significant when put in context of the relatively largelevels of NO_(x) emitted by an average FCC unit and the need in theindustry to continually decrease these emissions. Excessive emissionsare not only harmful to the environment, but can result is largeenvironmental fines for non-compliance, significantly more costlyprocesses than the present invention to meet regulation limits, and/orthe possibility of shutting down such units and the associated loss ofrevenues to meet non-compliance orders.

FIG. 4 shows the same data a plot of the same data as shown in FIG. 3,wherein the data was averaged and reformatted to more clearly illustratethe effects on NO_(x) as a function of NH₃/NO ratio for varying firstand second injection point H₂/NH₃ ratios. As can be seen in FIG. 4, thehigher H₂/NH₃ ratios at the second injection point unexpectedly showed atrend of lower NO_(x) levels than the lower H₂/NH₃ ratios at the secondinjection point. It can also be seen in the scatter data of FIG. 3, thatthe higher levels of H₂/NH₃ ratios at the second injection pointresulted in more consistent NO_(x) removal levels as compared to thoseachieved by the lower levels of H₂/NH₃ ratios shown in the same figure.

It should be noted that the readily-oxidizable gas utilized in FIGS. 2through 5 was hydrogen and that when reviewing FIGS. 3 through 5, thatonly hydrogen (with a steam carrier) was injected at second injectionpoint. Therefore, the “Pt. 2 H2/NH3” ratios shown in FIGS. 3 through 5is the ratio of the measured hydrogen injected at the second injectionpoint to the calculated unreacted ammonia remaining at the secondinjection point.

In addition to the significant improvement in NO_(x) reductionsdiscussed, it has been discovered that significant amounts of unreactedammonia remain in the process stream even at relatively low NH₃/NOratios (see FIG. 5 where the triangles mark the points of no secondaryhydrogen injection) and that addition of a secondary readily-oxidizablegas injection can significantly reduce the unreacted ammonia in theprocess stream. In FIG. 5, it can be seen, that with no secondary gasinjection, that the ammonia levels fluctuate severely from an average ofabout 80 vppm to absolute readings as high as 180 vppm. Introduction ofa readily-oxidizable gas at a secondary injection point reduces theammonia levels to below 40 vppm and eliminates the high fluctuation ofammonia levels in the regenerator off-gas stream. As noted prior, thesehigh ammonia levels can result in emissions problems as well assignificant equipment reliability and functionality problems, and wastewater treatment facility upsets and failures.

Since the amount of readily-oxidizable gas and reducing agent used aretypically a small percentage of the regenerator off-gas flow, typicallyless than about 0.5% by volume, based on the volume of the stream, it ispreferred to use only an effective amount of a readily available andrelatively inexpensive carrier material. Non-limiting examples ofcarrier materials include air and steam; however, any carrier materialthat does not have a deleterious effect on NO_(x) reduction, or whichitself contributes to undesirable emissions, can be used. Thus, it iscontemplated to mix effective amounts of reducing agent and/orreadily-oxidizable gas prior to mixing with a carrier material, orwithin the line that contains the carrier material. It is preferred thatthe reducing agent/readily-oxidizable gas mixture be injected into theline that conducts the carrier material.

By an effective amount of carrier material, it is meant an amount ofcarrier material that will adequately mix the reducing agent and/or thereadily-oxidizable gas with the process stream, i.e., maximize thecontact of the reagents with the NO_(x) sought to be reduced.

As previously stated, the regenerator off-gas also typically containscatalyst fines. These catalyst particles may be removed from theregenerator off-gas by any suitable means known in the art. However, thepresence of catalyst fines in the regenerator off-gas is believed toassist the NO_(x) reduction reaction. Thus, the presence of somecatalyst fines, although not necessary for the practice of the instantinvention, is preferred to assist the NO_(x) reduction reaction andreduce the amount of readily oxidizable gas that is needed.

In one embodiment of the present invention, effective amounts of areducing agent and a first readily-oxidizable gas, preferably with aneffective amount of carrier material, are injected directly into theregenerator's existing overhead line. Thus, the existing overhead linefunctions as the reaction zone for the NO_(x) reduction reaction,thereby eliminating the need to add costly processing equipment toeffectuate the present process. It is preferred that both the first andthe second injection points be located in the regenerator off-gas lineprior to any associated downstream processing equipment. A mostpreferred configuration is that the injection points be located as nearthe regenerator off-gas outlet as possible so that the highertemperatures near the regenerator outlet can be utilized, therebyreducing the amount of readily-oxidizable gas needed for a desired levelof NO_(x) reduction. It is also advantageous to maximize the residencetime of the reducing agent and readily-oxidizable gas in the NO_(x)reduction reaction.

In another embodiment, at least two or more injection points of areducing agent and a readily-oxidizable gas may be utilized prior to thefinal injection point wherein only the readily-oxidizable gas isinjected, preferably with a carrier gas, to achieve the low streamNO_(x) concentrations of the present invention. In this embodiment, themultiple injection points are preferably spaced such that theappropriate residence time between locations is achieved such that thedesired effect from the use of multiple injection locations is realized.As previously mentioned, it is advantageous to maximize the residencetime of the reducing agent and readily-oxidizable gas in the overheadline to complete the reactions.

In still another embodiment, it may be advantageous to have at least oneor more injection points of a reducing agent and a readily-oxidizablegas followed by one or more injection points wherein only thereadily-oxidizable gas is injected, preferably with a carrier gas, toachieve the low stream NO_(x). In this embodiment, it would be morepreferred if the readily-oxidizable gas was injected with the reducingagent at a relatively low readily-oxidizable gas to reducing agentratio, followed by more than one successive injections of areadily-oxidizable gas with successively higher readily-oxidizable gasto reducing agent ratios.

The above description is directed to several preferred means forcarrying out the present invention. Those skilled in the art willrecognize that other means and modifications, which are equallyeffective and obvious to those skilled in the art, could be devised forcarrying out the spirit of this invention.

EXAMPLES

The present NO_(x) reducing process was tested at a commercial FCCUunder normal operating conditions. The configuration of the twoinjection points and major equipment was as shown in FIG. 1 with theexception that an intermediate analyzer (shown in FIG. 1 as element(18)) was not installed. Therefore, the remaining unreacted NH₃ afterthe first injection point was not measured directly, but was based oncalculations from “ammonia slip” data compiled when operating theregenerator off-gas NO_(x) treatment configuration without the secondinjection point.

The following examples will illustrate the effectiveness of the presentprocess, but are not meant to limit the present invention.

Example 1

In this example, the NO_(x) reduction configuration for the FCCUregenerator off-gas was tested with only a single injection pointinjecting different ratios of a reducing agent (in this case NH₃) and areadily-oxidizable gas (in this case H₂). This first point injectionpoint is shown in FIG. 1 as point (6). In this example, the secondinjection point (7) was not utilized.

Data from the testing is shown in FIG. 2. Here it can be seen that theNO_(x) baseline readings for the regenerator off-gas ranged fromapproximately 60 to 90 ppm NO_(x) (see data plotted on y-axis). The dataplot in FIG. 2 illustrates the NO_(x) concentration as a function of theNH₃/NO ratio at differing H₂/NH₃ injection ratios. It can be seen fromthis graph, most importantly, that the lowest consistent NO_(x) levelsachievable are from about 40 to 60 ppm. Here it can also be seen thatonly at NH₃/NO levels from about 3 to about 8 can these NO_(x) levels ofabout 40 to 60 ppm be achieved.

It can also be seen in FIG. 5 that while operating at the NH₃/NO ratiolevels from about 3 to about 8 which are necessary to achieve maximumNO_(x) reduction, it has been found that the ammonia levels in theregenerator off-gas stream can be extremely high. This is shown by the“triangle” data points where the secondary injection point is 0 (i.e.,“Pt.2 H2/NH3=0”). These data points show that within this NH₃/NO range,the ammonia levels average about 80 vppm with individual readings ashigh as 180 vppm. It can also be seen that in this NH₃/NO range, thatthe ammonia levels fluctuate severely and therefore are not readilypredictable or controlled.

Example 2

In this embodiment of the present invention, the NO_(x) reductionconfiguration for the FCCU regenerator off-gas was tested with atwo-injection point system. A molar ratio of a reducing agent (in thiscase NH₃) to a readily-oxidizable gas (in this case H₂) of 1 to 3 wasinjected at the first injection point. At the second injection point,only the readily-oxidizable gas (in this case H₂) was injected. The datafrom these tests can be shown in FIGS. 3, 4, and 5. In these figures,the “Pt.2 H2/NH3” ratios shown in the legends are the ratio of themeasured hydrogen injected at the second injection point to thecalculated unreacted ammonia remaining at the second injection point.

FIG. 3 shows the NO_(x) in the treated regenerator off-gas stream as afunction of the NH₃/NO molar ratio at various injection point H₂/NH₃molar ratios. FIG. 4 shows the same data as FIG. 3, except the resultswere averaged and plotted at different NH₃/NO ratios. The data from FIG.4 shows that the higher H₂/NH₃ molar ratios at the second injectionpoint (i.e., the curves showing “Pt.2 H2/NH3” molar ratios of 14-16 and19-26) trend to lower NO_(x) concentrations as the NH₃/NO increases. Itcan also be seen in FIG. 3, that the lower H₂/NH₃ molar ratios at thesecond injection point (i.e., the curves showing “Pt.2 H2/NH3” molarratios of 0, 2-4 and 5-10) are less consistent in the regenerator NO_(x)readings than the higher H₂/NH₃ molar ratios at the second injectionpoint.

It can be seen from FIG. 3, that the NO_(x) in an FCCU regeneratoroff-gas is reduced to less than 50 vppm (parts per million by volume),preferably less than 40 vppm, more preferably less than 30 vppm. It canalso be seen from FIG. 4 that the average NO_(x) level without injectionis approximately 75 vppm and that NO_(x) levels of 30 to 40 vppm may beachieved with this embodiment of the present invention. This results inan overall NO_(x) reduction of 45 to 60 vol %.

FIG. 5 shows the NH₃ concentrations of the regenerator off-gas stream asa function of the NH₃/NO molar ratio. As can be seen, the higher H₂/NH₃molar ratios at the second injection point (i.e., the curves showing“Pt.2 H2/NH3” molar ratios of 5-10, 14-16 and 19-26) improve thereduction in ammonia in the regenerator off-gas that results from thefirst injection point. Therefore, this embodiment of the presentinvention results in improved NO_(x) reduction and lower ammoniaconcentrations (i.e., “slip”) in the regenerator off-gas of an FCCU.

It can be seen from FIG. 5, that without secondary injection of areadily-oxidizable gas (e.g. hydrogen), that that average NH₃ levels areabout 80 to 100 vppm. In comparison, this embodiment of the presentinvention results in NH₃ levels less than 30 vppm. This is a reductionof about 60% to about 70% in NH₃ levels than if a secondary injection ofa readily-oxidizable gas is not utilized in conjunction with the firstinjection of a reducing agent and a readily-oxidizable gas into the FCCUregenerator off-gas.

1. A non-catalytic process for reducing the NO_(x) concentration in theregenerator off-gas of a fluid catalytic cracking unit, comprising: a)forming a mixture of a reducing agent selected from ammonia, urea andmixtures thereof, and a first readily-oxidizable gas in effectiveamounts that will result in the reduction of the NO_(x) concentration ofthe regenerator off-gas by a predetermined amount; b) injecting saidmixture into said regenerator off-gas at a first injection point whereinthe regenerator off-gas is at a temperature between about 1200° F. and1600° F.; and c) injecting an additional amount of a secondreadily-oxidizable gas at a second injection point downstream of thefirst injection point in an amount effective to further reduce theamount of NO_(x) concentration of the regenerator off-gas and to reducethe concentration of the reducing agent in the regenerator off-gas. 2.The process of claim 1, wherein said readily-oxidizable gas is selectedfrom the group consisting of paraffinic, olefinic and aromatichydrocarbons and mixtures thereof, gasoline, fuel oil, oxygenatedhydrocarbons, formic and oxalic acids, nitrogenated hydrocarbons,sulfonated hydrocarbons, carbon monoxide, and hydrogen.
 3. The processaccording to claim 2, wherein said first readily-oxidizable gas and saidsecond readily-oxidizable gas are hydrogen.
 4. The process according toclaim 3, wherein said reducing agent is injected in a molar ratio ofabout 1 to about 10 moles per mole of NO.
 5. The process according toclaim 4, wherein said reducing agent is injected in a molar ratio ofabout 3 to about 8 moles per mole of NO.
 6. The process according toclaim 5, wherein said mixture comprises said first readily-oxidizablegas and said reducing agent in a molar ratio of about 1 to about 20moles of readily-oxidizable gas per mole of reducing agent.
 7. Theprocess according to claim 6, wherein said mixture comprises said firstreadily-oxidizable gas and said reducing agent in a molar ratio of about1 to about 8 moles of readily-oxidizable gas per mole of reducing agent.8. The process according to claim 7, wherein said mixture comprises saidfirst readily-oxidizable gas and said reducing agent in a molar ratio ofabout 1 to about 3 moles of readily-oxidizable gas per mole of reducingagent.
 9. The process of claim 8, wherein the reducing agent is ammonia.10. The process of claim 7, wherein said second readily-oxidizable gasis injected in a molar ratio of about 1 to about 40 moles of secondreadily-oxidizable gas per mole of unreacted reducing agent from saidfirst injection point.
 11. The process of claim 10, wherein said secondreadily-oxidizable gas is injected in a molar ratio of about 3 to about26 moles of second readily-oxidizable gas per mole of unreacted reducingagent from said first injection point.
 12. The process of claim 11,wherein said second readily-oxidizable gas is injected in a molar ratioof about 14 to about 26 moles of second readily-oxidizable gas per moleof unreacted reducing agent from said first injection point.
 13. Theprocess of claim 9, wherein said second readily-oxidizable gas isinjected in a molar ratio of about 3 to about 26 moles of secondreadily-oxidizable gas per mole of unreacted reducing agent from saidfirst injection point.
 14. The process of claim 1, wherein the finalNO_(x) concentration of said regenerator off-gas at a point downstreamof said second injection point is less than 50 vppm.
 15. The process ofclaim 14, wherein the final NO_(x) concentration of said regeneratoroff-gas at a point downstream of said second injection point is lessthan 40 vppm.
 16. The process of claim 11, wherein the final NO_(x)concentration of said regenerator off-gas at a point downstream of saidsecond injection point is less than 30 vppm.
 17. The process of claim12, wherein the final N_(x) concentration of said regenerator off-gas ata point downstream of said second injection point is less than 30 vppm.18. The process of claim 11, wherein the said reduction of the NO_(x)concentration of said regenerator off-gas is greater than 50 vol %. 19.The process of claim 12, wherein the said reduction of the NO_(x)concentration of said regenerator off-gas is greater than 50 vol %. 20.The process of claim 11, wherein said reducing agent is ammonia and theconcentration of the ammonia by vol % of said regenerator off-gas aftersaid second injection point is at least 60% lower than the concentrationof the ammonia by vol % of said regenerator off-gas between said firstinjection point and said second injection point.
 21. The process ofclaim 12, wherein said reducing agent is ammonia and the concentrationof the ammonia by vol % of said regenerator off-gas after said secondinjection point is at least 60% lower than the concentration of theammonia by vol % of said regenerator off-gas between said firstinjection point and said second injection point.
 22. The process ofclaim 20, wherein the concentration of ammonia of said regeneratoroff-gas after said second injection point is less than 40 vppm.
 23. Theprocess of claim 21, wherein the concentration of ammonia of saidregenerator off-gas after said second injection point is less than 40vppm.
 24. The process according to claim 23, wherein said reducing agentand said first readily-oxidizable gas are injected with a carriermaterial selected from steam and air.
 25. The process according to claim24, wherein catalyst fines from the regenerator vessel of an FCC unitare present in the regenerator off-gas.
 26. The process according toclaim 11, wherein said second readily-oxidizable gas is injected intosaid regenerator off-gas at a plurality of injection points downstreamof said first injection point wherein each injection point is located ata point further downstream than the prior injection point.
 27. Theprocess according to claim 12, wherein said second readily-oxidizablegas is injected into said regenerator off-gas at a plurality ofinjection points downstream of said first injection point wherein eachinjection point is located at a point further downstream than the priorinjection point.
 28. The process according to claim 26, wherein saidsecond readily-oxidizable gas injected at each of the plurality ofinjection points is injected at a higher molar ratio of secondreadily-oxidizable gas to unreacted reducing agent than the immediatelyprior upstream injection point.
 29. The process according to claim 27,wherein said second readily-oxidizable gas injected at each of theplurality of injection points is injected at a higher molar ratio ofsecond readily-oxidizable gas to unreacted reducing agent than theimmediately prior upstream injection point.